This invention relates to an apparatus for optically characterising thin layered material.
Such characterising that is non-destructive and can therefore be used in situ during manufacture or for checking finished products, enables knowing at least certain elements making up the analysed matter and possibly their concentration. It can also enable accessing the thickness of thin layers.
To perform characterising, we have known until now, on the one hand Raman spectroscopy devices, on the other hand reflectometers, photometers or ellipsometers, that may be spectroscopic.
We know that the Raman effect is caused by a sample lit at a given wavelength xcexe diffusing a Raman luminous beam at a wavelength xcexr close to xcexe whose intensity is very small with respect to that of the Rayleigh light which is diffused at the same wavelength xcexe as the lighting beam. In case when the Raman spectrometer is coupled with a microscope, the lighting beam is currently under a normal incidence with respect to the sample and the Raman diffusion is measured by its intensity and by its spectrum in a solid wide angle.
We also know reflectometry characterising. The purpose is then to light the sample under an angle that is often small (vastly different from the normal to the sample) and to analyse the light that is reflected specularly by the sample. We are then more particularly interested in the luminous intensity in the case of photometry and in the amplitude of the various components of the polarised light in the case of ellipsometry.
We know, in particular, phase-modulated ellipsometry in which a modulator acts on the polarisation state of the incident luminous beam, the spectroscopic ellipsometry in which the wavelength spectrum of the reflected light is analysed and the modulated reflectometry that takes into account the effect of the modulation generated by a periodic external excitation, for example electric or optic excitation, acting on the sample. A modulated spectroscopic ellipsometer is for instance described in the European patent EP-0.663.590 to which reference can be made.
Each one of these major alternatives of optical characterising of a sample exhibits its own advantages.
Generally speaking, macroscopic response of a thin layered material to an electromagnetic excitation by a tensor xcex5(xcfx89) where xcfx89 is the frequency of the electromagnetic excitation. In the case of an isotropic solid, this tensor xcex5(xcfx89) is reduced to a scalar and we obtain the relationship D=xcex5o xcex5E where xcex5o is electric permittivity of vacuum, D is the electric displacement vector and E is the applied electric field.
Polarisability xcex1 is then defined on the base of the local bipolar moment p (per atom or group of atoms). Indeed, p is linked with the local electric field Eloc (itself function of the external electric field E by the relationship: p=xcex1xcex5o Eloc.
The macroscopic dipolar moment per volume unit or polarisation vector is given by the formula: P=Np where N represents the space density of dipoles. Polarisation is linked with the other macroscopic quantities by the relation: P=xcex5o (xcex5xe2x88x921) E.
Reflectometry, more particularly, spectroscopic ellipsometry, enables accessing the dielectric function xcex5(xcfx89). In the range of wavelengths from ultraviolet to the visible, absorption is often dominated by electronic transitions (it is for example the case in semiconductors). In the infrared range, ellipsometry is sensitive to vibration absorption, i.e. dipole excitation. The thickness probed may vary considerably in relation to the wavelength as in the case of semiconductors that are generally very absorbing in the ultraviolet and quasi transparent in the infrared.
The Raman diffusion is, for its own part, sensitive to polarisability variations in the presence of excitations xcex94xcex1(xcfx89).
It can be noted that, as regards the determination of related physical values, ellipsometry and Raman diffusion are techniques of different natures. In particular, from the viewpoint of quantum mechanics, the efficient sections of certain vibrations could be very different in one case and in the other.
Reflectometric measurement is conducted in a specular and elastic fashion (conservation of the wavelength), generally in reflection. Consequently, it is sensitive to interference phenomena that enable measuring thicknesses of thin layers. More generally, ellipsometry is well suited for characterising a multilayer material (which exhibits thickness divergences). In usual applications of ellipsometry, the angle of incidence varies between 55 and 80xc2x0 approximately, which corresponds to the Brewster angles of most materials and provides optimal sensitivity. Two wavelength ranges are used generally: the first is said xe2x80x98visible ultravioletxe2x80x99, extending from the near ultraviolet (0.25 xcexcm) to the near infrared IR (1.7 xcexcm) and the second, called xe2x80x98infraredxe2x80x99, in the more remote infrared from 2.5 to 12 or 16 xcexcm approximately. Measurements with higher wavelengths are difficult because of experimental limits imposed by the sources and the detectors.
xcex5(xcfx89) is represented by a complex number whose determination calls generally for the measurement of two independent parameters as this can be made in ellipsometry. However, photometry, pending the use of so-called Kramers-Konig relationships, can also enable measuring xcex5(xcfx89). Modulated reflectometry techniques measure the variation of xcex5(xcfx89) in the presence of an external excitation, which brings complementary information. In particular, in semiconducting materials, the modulated external excitation generates loaded carriers that concur to that measurement.
Conversely, the Raman diffusion is inelastic. The measurement is then generally conducted in normal incidence; whereas a laser that emits a ray in the ultraviolet, the visible or the near infrared range provides the excitation. The Raman photons are collected under a solid wide angle at wavelengths close to those of the incident light. We therefore measure spectroscopically a positive or negative difference in wavelengths between the exciting ray and the Raman spectrum. By comparison, the remote reflectometry infrared corresponds in Raman to the wavelengths closest to the exciting wavelength. Such measurements are therefore technically easier in Raman spectrometry. It should be underlined that the characterised thickness is linked with the absorption of the material at the wavelength of the incident light that is hardly modifiable in Raman for a given material.
Thin films have been subject to Raman spectroscopy surveys as of the end of the sixties. It has been suggested first of all to study thin layers deposited on a metal surface. Lit under an angle of incidence of 70xc2x0, the Raman flux obtained exhibited a maximum intensity around 60xc2x0. It has then been proposed to use a thin layer as an optic wave-guide to which a light flux was coupled by means of a prism or of a grating. Under strict conditions of angle of incidence and of polarisation, one or several electric or magnetic transverse modes can be propagated in the film while creating therein a Raman flux whose intensity can reach up to two thousand times the flux intensity usually generated by backscattering. In such a case, the minimum thickness of the film, linked with the excitation wavelength, cannot be smaller than a few nanometres. We can go below this limit only while resorting to multilayer structures. Anyway, all these methods call for particular preparation of the film or of the thin layer, on a specific support, which implies tooling and adjustments incompatible with the survey of industrial materials and even more with in situ or real-time measurements, during the implementation of a method of manufacture.
These presentations of the measurements by Raman effect on the one hand, and by reflectometry on the other hand, ellipsometric or photometric reflectometry, as they outline that we obtain different effects, hence different sources of knowledge and characterising, also show the difficulties that may be encountered especially when the material is thinly layered.
The realisation of the measurements on the same sample, by reflectometry and by Raman spectroscopy, exhibits other difficulties. Indeed, the different angles of incidence for one or the other of these measurements, the wavelengths also different call for optic apparatuses that are especially suited to each of the techniques and do not allow using the same source for lighting nor the same wavelength detection system for reception, in one and the other case.
Moreover, it has been considered for a long time that simultaneous implementation of both these techniques was liable to produce parasitic effects damaging the quality of each measurement. Thus, the backscattering Raman spectrometry measurements imply to light the sample with luminous intensities that can modify the sample while inducing annealed, crystallised or effused (extraction of hydrogen atoms for example) matters and risk to compromise reflectometric measurement.
It should be noted especially that the sizes of the zones lit by the sample during either of these measurements could be quite different.
Using a single type of measurement is for instance described in the document WO-97/05473, which relates to optic microsamples and methods for spectral analysis of materials. In a particular embodiment, a multimode optic fiber cuts a light source and a sample. The sample is also coupled to a sensor, which senses the light reflected by the sample lit, according to an angle predetermined with respect to the direction of the light.
Still, as regards the realisation of the Raman and reflectometric measurements on zones superimposed with a thin layer, simultaneously, exhibits great advantages, on the one hand directly as regards characterising materials, on the other as regards checking preparation methods that may implement such characterising modes. As measurements can be made simultaneously in real time, it is possible to follow the variations and the evolution relative to the velocity of the parameters measured. We obtain thus a very detailed physicochemical description of the layers, as it is illustrated below.
The aim of this invention is therefore to solve the various difficulties mentioned above and to suggest an apparatus that enables characterising a thin layer by backscattering Raman effect, without any risk of modifying the layer during measurement.
It is another aim of the invention to suggest an apparatus enabling simultaneous characterising by Raman effect and by reflectometry.
Reflectometry encompasses, as we have stated above, photometric measurements in which only the luminous energy is measured and the ellipsometric measurements in which the different polarised components of the light flux are considered. In both cases, reflectometry can be spectroscopic, i.e. spectral analysis of the measured flux is realised.
To this end, the invention concerns an apparatus for characterising a thin-layer material by backscattering Raman spectrometry comprising a frame, a monochromatic excitation laser source, optical means directing a light flux toward the material to be characterised, and means for collecting and selecting the light diffused by Raman effect.
In optical means directing the excitation laser flux toward the sample, there exists between the laser and the sample a means homogenising the distribution of energy per surface unit, over a minimum surface of some tens of square micrometers.
According to the invention, the apparatus comprises means for reflectometric measurement, integral with the Raman measuring means, whereas this reflectometric measuring means comprises reflectometric excitation means directed on the same zone of the sample as the Raman excitation means.
The apparatus of the invention can thus be used;
either for pure Raman spectrometry measurements,
or for pure reflectometric measurements,
or for combined Raman spectrometry and reflectometric measurements.
In different particular embodiments, each exhibiting their own advantages and liable to be implemented in numerous technically possible combinations:
the reflectometric measuring means are photometric measuring means;
the reflectometric measuring means are ellipsometric measuring means;
the means homogenising the distribution of energy comprises a multimode fiber;
the diameter of the fiber is suited to the surface of the sample that must be lit;
the fiber is interchangeable; thus the diameter lit can be changed using barrels with fibers of different diameters;
the apparatus comprises several barrels enabling to light the sample over a focusable zone over the slot of a spectrometer or to suit the Raman excitation surface to the surface analysed by photometry;
the reflectometric measuring means are modulated reflectometry measuring means;
the modulation is optical;
the modulation is electric;
the modulation is ensured by the Raman excitation source;
the reflectometric source is coupled to the remainder of the apparatus by an optical fiber;
at least one of the Raman and reflectometric receivers is connected to the remainder of the apparatus by an optical fiber.